Field of the Invention
The present invention relates to an exhaust gas recirculation apparatus for an engine to allow part of exhaust gas discharged from an engine to an exhaust passage to flow in an intake passage to recirculate back to the engine.
Related Art
Conventionally, a technique of the above type is employed in a vehicle engine, for example. An exhaust gas recirculation (EGR) apparatus is arranged to introduce part of exhaust gas after combustion, which is discharged from a combustion chamber of an engine to an exhaust passage, into an intake passage as EGR gas through an EGR passage so that the exhaust gas is mixed with intake air flowing in the intake passage and flows back to the combustion chamber. The EGR gas flowing in the EGR passage is regulated by an EGR valve provided in the EGR passage. This EGR can reduce mainly nitrogen oxide (NOx) in the exhaust gas and improve fuel consumption during a partial load operation of the engine.
Exhaust gas from the engine contains no oxygen or is in an oxygen lean state. Thus, when part of the exhaust gas is mixed with the intake air by EGR, the oxygen concentration of the intake air decreases. In a combustion chamber, therefore, fuel burns in a low oxygen concentration. Thus, a peak temperature during combustion decreases, thereby restraining the occurrence of NOx. In a gasoline engine, even when the content of oxygen in intake air is not increased by EGR and a throttle valve is closed to some degree, it is possible to reduce pumping loss of the engine.
Herein, recently, it is conceivable to perform EGR in the entire operating region of the engine in order to further improve fuel consumption. Realization of high EGR is thus demanded. To realize the high EGR, it is necessary for conventional arts to increase the internal diameter of an EGR passage or increase the opening area of a flow passage provided by a valve element and a valve seat of an EGR valve.
JP-A-2012-7505 discloses an EGR apparatus to be provided in a supercharger-equipped engine. This supercharger includes a compressor provided in an intake passage and a turbine provided in an exhaust passage and is configured to rotate the turbine by exhaust gas flowing in the exhaust passage, thereby integrally rotating the compressor, to increase the pressure of intake air in the intake passage. The EGR apparatus is a low-pressure loop type EGR apparatus in which an EGR passage is provided between an exhaust passage downstream of the turbine and an intake passage upstream of the compressor. An EGR valve provided in the EGR passage is configured to be controlled by an electronic control unit (ECU) according to an operating condition of an engine. This EGR apparatus is arranged to forcibly narrow the EGR valve down to a predetermined low opening degree once at the time of engine deceleration in order to prevent misfire of the engine during deceleration, and hold the narrowed state. This control is performed to temporarily increase pressure loss in the EGR passage during engine deceleration and also prevent a larger amount of EGR gas than assumed from flowing from the EGR passage into the intake passage.
Herein, one of the EGR valves for use in the EGR apparatus is already known as a motor-driven EGR valve arranged to open and close a valve element with respect to a valve seat by a step motor or a DC motor to control the valve element to a small opening degree. This kind of EGR valve is disclosed in for example JP-A-2007-309115.